In exploration for hydrocarbons, it is important to make accurate measurements of geologic formations. The geologic formations below the surface of the earth may contain reservoirs of oil and gas. The geologic formations may include formation bedding planes and various structures. In a quest for oil and gas, it is important to know about the location and composition of the formation bedding planes and the various structures. In particular, it is important to know about the geologic formations with a high degree of accuracy so that reservoir production is optimized. Measuring properties of the geologic formations provides information that can be useful for locating the reservoirs of oil and gas. Typically, the oil and gas are retrieved by drilling boreholes into the subsurface of the earth. The boreholes also provide access for taking measurements of the geologic formations.
Well logging is a technique used to take measurements of the geologic formations from the borehole. In one embodiment, a “logging instrument” is lowered on the end of a wireline into the borehole. The logging instrument sends data via the wireline to the surface for recording. Output from the logging instrument comes in various forms and may be referred to as a “log.” Many types of measurements are made to obtain information about the geologic formations. Some examples of the measurements include gamma ray logs, nuclear magnetic resonance logs, resistivity logs, pressure logs, and sonic or acoustic logs.
With today's sophisticated drilling and logging techniques, it is important to have an accurate orientation of the logging tool in the borehole. For example, the boreholes may be deviated from a vertical plane and even horizontal. As one might imagine, in a horizontal borehole, it is important to know whether one is taking measurements of the formations above or below the borehole. Even in vertical boreholes, it is important to know the orientation of certain measurements so that the orientation of the formations with respect to the borehole may be discerned.
Typically, data from several logging tools are analyzed side-by-side to form a composite picture of the formations. Even small errors in the orientation of the logging tool can corrupt logging data. An assumption that the logging instrument is moving smoothly through the borehole is not always valid due to rugose and sticky borehole conditions. Additionally, tool centralizers and decentralizers may keep the logging tool from moving smoothly and contribute to disorientation.
Sensors are needed to determine the orientation of the logging tool. The sensors may also be used to measure acceleration due to external perturbations acting upon the logging tool. Data from measuring the acceleration can be used to determine the orientation of the logging tool.
It is also important to measure pressure at various depths within the borehole. If the pressure is not kept under control, then an uncontrolled release of oil and gas to the surface (known as a “blowout”) can result. The blowout can cause personal injuries, drilling rig damage, environmental damage, and damage to underground reservoirs. Pressure sensors are needed to monitor the pressure within the borehole.
Conditions hostile to sensors within the logging instrument exist in the boreholes. For example, high temperatures and pressure may cause the sensors to fail. Failure of sensors such as the accelerometers and pressure sensors in the borehole can lead to wasting a significant amount of resources.
What are needed are an apparatus and a method for making accurate measurements of acceleration and pressure in the borehole.